1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image (electrostatic latent image) on a photographic recording medium such as photosensitive material, photosensitive member, or the like by using an optical write head having a light shutter array made of PLZT or a LED array.
2. Description of Related Art
Conventionally, various kinds of optical write heads which turn on and off light for each pixel with use of a light shutter array made of PLZT or an LED array have been used to form images (electrostatic latent images) on a silver-salt print sheet or film or an electrophotographic photosensitive member. Also, in order to realize an image forming at high speed, an image forming apparatus which has three optical write heads has been disclosed (refer to U.S. Pat. No. 5,488,450).
In the image forming apparatus with three optical write heads, a tandem method is adopted; which is, with three optical write heads arranged in row in the sub scanning direction, an image is composed and recorded on a recording medium being carried flat by exposing one by one with specified time lag. However, the apparatus of tandem method has problems: it is difficult to carry the recording meidum flat at high accuracy; and, an extra control is needed in order to synchronize three exposure positions, and operation of the control is complicated.
On the other hand, an optical write head, in order to form high-quality images without unevenness, needs to be subjected to measurement of the quantity of light outputted from each element and to correction in quantity of light according to the measurement result. Conventionally, an optical write head is fitted in a jig before it is built in an image forming apparatus, and the quantity of light outputted from each element is measured by a light-quantity measuring device with a photosensor; then, correction date are produced based on the result of the measurement. However, those correction data are effective only in the early time of building the optical write head in an apparatus, and are not useful when the output characteristic of each light element changes because of aging and/or a change in environmental conditions. That is, a real-time light-quantity correction for three optical write heads has not been achieved in the present condition.